It is well known that under normal service conditions the insulation of a.c. and d.c. electric equipment is exposed to the impact of operating voltage of the electric network system.
However, for a variety of different reasons one or another part of the electric system is liable to undergo a momentary increase in voltage being much in excess of that used under normal service conditions, which consequently gives rise to overvoltage phenomena. Should the amplitude of this overvoltage reach a substantial value it may present a hazard to the insulation of the electric installations of stations, substations and power transmission lines, in particular such overvoltage arising on the power transmission line are capable of damaging the insulation of the most expensive equipment items, i.e. electric machines, transformers, reactors, switching apparatus.
The suppression of the level of overvoltages arising on the power transmission line is accomplished by overvoltage protective devices, the application of which has now become vitally indispensible to the creation of power transmission lines of the highest voltage ratings.
It should be noted that the operational reliability of electric equipment is largely determined by and dependent on the operational reliability of overvoltage protective devices.
Known in the prior art is an overvoltage protective device (see the U.S. Pat. No. 3,805,114 issued in 1974) comprising a column of non-linear resistors mounted within an insulating housing. Between the non-linear resistors and the inner surface of the insulating housing there is provided a gap. The non-linear resistors are made from zinc oxide-based material.
The non-linear resistors are characterized by a non-linear voltage-current relationship and act as a low resistance to the flow of high-magnitude overvoltage-induced currents, thus limiting the voltage across the terminals of the overvoltage protective device, and as a high resistance under normal service conditions, thus limiting the magnitude of current flowing through the device from the electric network.
Under normal service conditions the current of a small magnitude supplied by the electric network passes through the overvoltage protective device in a continuous manner.
Upon the occurence of overvoltage in the electric network as a result of the high non-linearity inherent in the resistors the passage of large-magnitude overvoltage-induced currents through the overvoltage protective device results in a pronounced increase in voltage at the point of connection of the overvoltage protective device to the electric network. The overvoltage across the equipment connected to the electric network in parallel with the overvoltage protective device is thus limited.
Upon cessation of the overvoltage impact on the electric network a sharp increase in resistance of the non-linear resistors takes place, due to which through the overvoltage protective device is restored the passage of the original magnitude of the current from the electric network typical for normal service conditions.
However, the constant passage of current through the device is attended with the release of heat energy which should be dissipated to reduce the temperature of the non-linear resistors.
The air gap between the lateral surfaces of the non-linear resistors and the inner surface of the porcelain insulating housing affects the satisfactory heat removal from the non-linear resistors. Meanwhile, the protracted exposure of the non-linear resistors to voltage, particularly at elevated temperatures of the non-linear resistors results in the ageing of the material used for their fabrication, which leads in turn to a gradual decrease in resistance of the non-linear resistors and, therefore, to a build-up of the current flowing therethrough under the action of the electric network voltage. This may eventually cause a disturbance in the thermal balance of the overvoltage protective device and its complete failure, thus reducing the operational reliability of the device under consideration.
Furthermore, upon the occurrence of a short-circuit electric arc in the above air gap accompanying sparkover of the resistors there is generated a large amount of gas contacting directly the inner surface of the insulating housing, and inasmuch as the air gap is too small in transverse direction the gas being formed can not be removed rapidly from the overvoltage protective device, which results in an abrupt increase in pressure of the gas thus formed, creating heavy explosion hazards in the porcelain insulating housing. Moreover, direct contact between the electric arc and the gas having a high temperature brings about excessive heating of the insulating housing, which may also cause its destruction. All this taken together impairs severely the operational reliability of the above-described overvoltage protective device.
Also known in the prior art is an overvoltage protective device (see the U.S. Pat. No. 4,100,588 and the specification of a F.R.G. application No. 2,804,617 issued Sept. 21, 1978, U.S. claims priority of Mar. 16, 1977) comprising an insulating housing accommodating thereinside at least one column of non-linear resistors.
The greater portion of the longitudinal space between the column of non-linear resistors and the inner surface of the insulating housing is filled with thermally conductive dielectric material which closely envelops the column of non-linear resistors and is in contact with the inner surface of the insulating housing.
The thermally conductive dielectric material is a silicone rubber including alumina as a filler.
The lesser, not filled with the thermally conductive dielectric portion of the longitudinal space between the vacant inner surface of the insulating housing and the surface of the thermally conductive dielectric bulk material defines a gas vent channel arranged along the length of the column of the non-linear resistors.
The thermally conductive dielectric material improves heat contact between the non-linear resistors and the insulating housing, which ensures more effective removal of heat from the non-linear resistors as compared to the non-linear resistor heat removal system employed in the overvoltage protective device described hereinabove.
The gas vent channel allows an unhindered discharge of gas from the device, the pressure of the gas formed being less than the pressure which the insulating housing can withstand. This reduces the possibility of explosive destruction of the insulating housing.
However, similarly to the above-mentioned device, this prior art overvoltage protective device is deficient in that the short-circuit electric arc and the gas formed are also in direct contact with the inner surface of the insulating housing, which results in excessive heating of the insulating housing and its consequent cracking, this decreasing the mechanical strength of the insulating housing, whereby the pressure of the gas being formed may thus be sufficient to cause explosion of the insulating housing. Hence, this factor affects adversely the operational reliability of the overvoltage protective device.
Furthermore, the amount of labor and time taken by the procedure of filling the insulating housing with the thermally conductive dielectric material should be also noted. Thus, the thermally conductive dielectric material is poured into the insulating housing subsequent to the mounting of the column of non-linear resistors therein, whereafter the housing is turned over onto its lateral surface. Upon solidification and self-levelling of the thermally conductive dielectric material inside the insulating housing there is formed a gas vent channel. As the process takes place, the thermally conductive dielectric material should be prevented from penetrating into the space between the non-linear resistors.